1. Field of the Invention
The present invention relates to the production of hydrocarbon products from oil shale deposits, and, more particularly, to the in situ processing of oil shale ore to recover said hydrocarbon products.
2. Description of the Prior Art
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for the recovery of hydrocarbon and mineral products therefrom. The term "oil shale" is widely used to refer to a layered sedimentary formation containing an organic waxy material known as kerogen. While kerogen is practically immobile within the oil shale, when the oil shale is heated over a period of time and to an appropriate temperature, the kerogen decomposes to produce gaseous and liquid hydrocarbon products. Additionally, it has been found that some oil shale deposits contain substantial quantities of other valuable minerals, such as nahcolite, a naturally occurring sodium bicarbonate, and dawsonite, a sodium-aluminum compound, recovery of which will help to make recovery of the hydrocarbon products more economically feasible. The term "oil shale ore" is used herein to include such mineral-bearing shales.
Deposits of oil shale ore have not been exploited to a significant extent as a source of oil due to the relatively high cost of mining and recovering the oil, and the environmental considerations involved in such operations. However, there have been four basic methods proposed for processing the oil shale ore, namely: the pure in situ method; the modified in situ method; the surface retort method; and the multi-mineral method. At the present time, it is believed that the pure in situ method is still experimental in nature.
On the other hand, the modified in situ method is very popular with the industry, because it represents an attractive concept for low-cost production of shale oil by underground pyrolysis. With the modified in situ method, an underground retort is formed by removing a portion, e.g., 15 to 30 percent, of the oil shale ore in the retort zone to create a void space. This ore, which is mined by conventional techniques, is transported to the surface. Explosives are then disposed in the ore deposit and the underground retort zone is created by detonating the explosives to rubblize the remaining oil shale ore, which then fills the retort zone. The rubblized oil shale ore is then subjected to pyrolysis by igniting the ore and sustaining the burn by pumping air into one end of the chamber and withdrawing gases from the other. As the burn front advances through the retort zone, the hot combustion gases pyrolize the kerogen in the oil shale to form hydrocarbon vapors. These vapors are cooled as they move toward the base of the chamber, where they contact the cooler ore and condense into shale oil. The oil may then be pumped from the base of the retort and piped to the surface.
The modified in situ method has two shortcomings: channeling and water entry. The phenomenon of channeling occurs due to the presence of fine particles, i.e., the "fines", in the oil shale rubble. The permeability of the portions of the rubblized bed of ore containing the "fines" is lower than the permeability of the portions of the bed containing the larger particles of oil shale ore. The burn front advances more rapidly where the bed has a higher permeability, and the areas of the retort zone comprising "fines" are bypassed and not retorted. Accordingly, substantial quantities of shale oil might not be recovered, thereby resulting in an inefficient and less economical process.
As noted above, a second problem with the modified in situ method arises by virtue of water entry into the retort zone. Water entry is commonly encountered because joints and fractures are abundant in many of the oil shale ore deposits. If a particular area is water-bearing, the detonation of explosives may permit water to flow into the retort. The water is costly to remove, and causes inefficient retorting when it contacts the burn front.
There are several methods of surface retorting, e.g., as disclosed in U.S. Pat. No. 3,025,223 to Aspergren, et al. While surface retorting techniques have been utilized, they are not only labor and material intensive, but also present environmental difficulties which may be costly to overcome. The economics of surface retorts have not yet been proven in commercial scale, and they are highly capital intensive.
With the multi-mineral oil shale process, nahcolite, shale oil, alumina, and soda ash may be obtained from the mineral-bearing oil shale ore. The multi-mineral process is a surface technique and may employ a circular grate as the pyrolysis mechanism. For a more detailed explanation of this process, reference should be made to U.S. Pat. No. 3,821,353 to Weichman and U.S. Pat. No. 4,082,645 to Knight, et al. While the multi-mineral process has many desirable characteristics, the surface nature of the operation makes it labor intensive and subject to the environmental considerations mentioned above.